1. Field of the Invention
This invention relates to an optical recording method by use of an optical recording medium, particularly to an optical recording method advantageous in aspect of energy saving, which is capable of high speed pit recording of high density and high sensitivity even when a semiconductor laser of low output is used. Further, the present invention relates to an optical recording method which has rendered three-value recording possible with one sheet of an optical disc by using color change recording and pit recording in combination, namely by use of two kinds of dots.
2. Description of the Related Art
Recently, optical recording media such as optical disc, optical tape, optical card, etc. (hereinafter called comprehensively optical discs) are of primary interest among office automations. For example, since an optical disc is capable of recording and storing a large amount of documents and literature in one sheet, they have the advantage that documents in an office can be pigeonhold or managed with good efficiency.
The recording layer to be used in such optical disc technique can retain a high density of information in the form of a spiral or circular track of small dots (e.g. about 1 .mu.m) which are optically detectable.
A typical example of the disc to be used in this optical disc technique is constituted of a recording layer comprising a material sensitive to laser provided on a substrate. For writing of information into this disc, the laser converged onto the laser sensitive layer (recording layer) is scanned to form dots only on the surface irradiated with the laser beam, thereby forming the dots in the form of a spiral or circular track corresponding to the recording information. Thus, the laser sensitive layer is capable of forming optically detectable dots by absorption of laser energy. For example, according to the heat mode recording system, the laser sensitive layer can absorb heat energy to form dots comprising small concavities (pits) by evaporation or melting on that site. Alternatively, according to another heat mode recording system, it is possible to form dots comprising optically detectable color changed portions at that site irradiated by absorption of the laser energy.
The information recorded on the optical disc is detected by reading the optical change of the portion where dots are formed from the portion where no dot is formed by scanning a laser along the track. For example, when a disc with a constitution having a recording layer provided on the reflective surface of a substrate, a laser is scanned along the track and the energy reflected by the disc is monitored by a photodetector. During this operation, in the case of pit recording, the output of the photodetector will be lowered at the position where no pit is formed, while the laser beam will be sufficiently reflected by the reflective surface at the position where pits are formed to make the output of the photodetector greater.
As the recording layer of such optical disc, investigations have been made concerning various kinds of materials employing primarily inorganic substances, including metal thin films such as aluminum vapor deposited film, etc., bismuth thin films, tellurium oxide thin films or chalcogenide type amorphous glass films, etc., but those employing organic materials are now attracting attention in aspect of lower cost as well as ease in manufacturing.
As the organic material to be used in optical recording medium, U.S. Pat. Nos. 4,501,801, 4,500,978 and 4,577,208 disclose organic coatings containing pyrylium dyes, croconic methine dyes, polymethine compounds, etc., of specific structures having good thermal stability. Since the organic coatings containing these compounds can generate heat by absorption of radiation in the radiated wavelength region of semiconductor laser, it has been known that the so called heat mode recording to form pits by laser energy can be practiced.
However, in the pit recording system of the prior art, although there is the advantage that recording is clear, that is, contrast is good, as compared with the recording system which forms dots by color change, there have been involved the drawbacks that more energy is required for recording. Further recording speed is slower. Further, since pits are formed physically on the surface of a recording medium, the recording medium surface at the initial stage is required to be sufficiently smooth and at the same time sufficient care is needed so that the recording medium surface is not damaged even after recording. It has been also difficult to practice recording at high density and high sensitivity.
On the other hand, since the heat mode system is a two-value recording whether a dot is formed or not, it is limited in practicing recording of higher density.